The present invention relates to systems and apparatuses for stabilizing the high-temperature process tubes of a fired heater, furnace, heat exchanger or other device that utilizes high-temperature process tubes. However, systems and apparatuses used in accordance with the present invention are particularly well suited and advantageous to the reactor tubes of furnaces used for the cracking of a variety of hydrocarbon feedstocks by pyrolysis to ethylene and other valuable olefinic gases. Accordingly, by way of illustration, but not limitation, the present invention will be described and explained in the context of that process.
Cracking furnaces long have been used in the process of cracking a variety of hydrocarbon feedstocks to ethylene and other valuable olefinic gases. There are at least several thousand such furnaces located in world refineries and petrochemical plants. U.S. Pat. Nos. 2,671,198, 3,407,789, 3,671,198, 4,342,642, 4,499,055 and 5,427,655 describe basic designs of short-residence time/high temperature cracking furnaces. In general, cracking furnaces vary in size and style but all contain reactor tubes, which transport the feedstock being heated and processed. The sensible heat and the heat of cracking are supplied by radiant heat from burners located on the floor and/or walls of the firebox of the furnace. This heat transfers through the reactor tubes into feedstock that flows therewithin.
Given the relatively high temperatures to which the reactor furnace tubes are exposed, metallic materials have been preferred for construction of reactor lines. Recent conventional reactor lines have been constructed of nickel-containing alloys, however, varying the materials used for reactor lines are found in the prior art. See, e.g., Winkler et al., U.S. Pat. No. 2,018,619, describing an apparatus that uses reactor tubes made from silicon powder; and European Patent Application EP 1 018 563 A1 describing constructing a portion of a heating furnace tube with a material comprising a rare earth oxide particle dispersion (“ODS”) iron alloy.
The length of reactor furnace tubes varies and generally may range from about 10 feet to about 400 feet in length. Further, reactor furnace tubes may take many shapes. Although the present invention is particularly well suited and advantageous to reactor furnace tubes that are coiled in a serpentine shape or comprised of a series of u-shaped tubes, other configurations are within the contemplated scope of the present invention. Among the problems relevant to the present invention associated with reactor furnace tubes is the movement of the tubes due to harmonics, fluid momentum, thermal expansion and/or other forces. Such movement is sometimes referred to “swing.” Movement of the tubes introduces harmful stresses in the tubes and their welds, which distort the shape of the tubes, reduce the useful life of the tubes and create a safety risk of tube or weld breakage. Such movement also disturbs the alignment of the tubes with the burners of the furnace, which is sometimes referred to as “shadowing,” i.e., one tube blocking the radiant heat from reaching another tube. Misalignment also can reduce the efficiency of the process by allowing the reactor tubes to get too close or too far from the burners thereby causing inconsistent heat transfer. In addition, misalignment of the reactor furnace tubes can cause an increase in coke formation, a deleterious by-product of the process, within the tubes. The deposition of coke on the insides of the reactor furnace tubes constricts the flow path for the feedstock, causing an increased system pressure drop, and a decrease in the furnace capacity. Additionally, the coke deposition on the inside of reactor furnace tubes decreases the heat transfer of the radiant heat from the radiant burners through the tube wall to the hydrocarbons flowing through the tubes, which results in a decrease in the cracking yield. Coking in conventional reactor furnace tubes is major cause of furnace shutdown.
To date, manufacturers have welded locator pins to the return bends of u-shaped reactor furnace tubes in an attempt to limit their movement. Despite such efforts, the problems of “swing,” “shadowing,” misalignment and their adverse effects continue and there is no prior art that teaches or suggests systems or apparatuses for stabilizing reactor furnace tubes using a framework of structural components that effectively limits the movement of the reactor furnace tubes.